[Illustration:  Fig. 288.  Double Clearing-Out Drops]

Recently there has been a growing tendency on the part of some manufacturers to control their clearing-out signals by means of relays associated with cord circuits, these signals sometimes being ordinary clearing-out drops and sometimes incandescent lamps.

[Illustration:  Fig. 289.  Relay-Controlled Clearing-Out Drop]

In Fig. 289 is shown the cord circuit sometimes used by the L.M.  Ericsson Telephone Manufacturing Company.  A high-wound relay is normally placed across the cord and this, besides having a high-resistance and impedance winding has a low-resistance locking winding so arranged that when the relay pulls up its armature it will close a local circuit including this locking winding and local battery.  When once pulled up the relay will, therefore, stay up due to the energizing of this locking coil.  Another contact operated by the relay closes the circuit of a low-wound clearing-out drop placed across the line, thus bridging it across the line.  The condition of high impedance is maintained across the cord circuit normally while the subscribers are talking; but when either of them rings off, the high-wound relay pulls up and locks, thus completing the circuit of the clearing-out drop across the cords.  The subsequent impulses sent from the subscribers’ generators operate this drop.  The relay is restored or unlocked and the clearing-out drop disconnected from the cord circuit by means of a key which opens the locking circuit of the relay.  This key is really a part of the listening key and serves to open this locking circuit whenever the listening key is operated.  The clearing-out drop is also automatically restored by the action of the listening key, this connection being mechanical rather than electrical.

Recall Lamp:—­The Monarch Company sometimes furnishes what it terms a recall lamp in connection with the clearing-out drops on its magneto switchboards.  The circuit arrangement is shown in Fig. 290, wherein the drop is the regular double-wound clearing-out drop like that of Fig. 284.  The armature carries a contact spring adapted to close the local circuit of a lamp whenever it is attracted.  The object of this is to give the subscriber, whose line still remains connected by a cord circuit, opportunity to recall the central office if the operator has not restored the clearing-out drop.

[Illustration:  Fig. 290.  Cord Circuit with Recall Lamp]

Lamp-Signal Type. There has been a tendency on the part of some manufacturing companies to advocate, instead of drop signals, incandescent lamp signals for the cord circuits, and sometimes for the line circuits on magneto boards.  In most cases this may be looked upon as a “frill.”  Where line lamps instead of drops have been used on magneto switchboards, it has been the practice to employ, instead of a drop, a locking relay associated with each lamp, which was so arranged that when the relay was energized by the magneto current from the subscriber’s station, it would pull up and lock, thus closing the lamp circuit.